This invention relates generally to ground fault detectors for direct current power supplies and more particularly, it relates to a ground fault detector which senses the possibility of direct current leakage or other possible d.c. fault currents by continuously measuring the a.c. electrical impedance between a high voltage d.c. circuit and chassis ground. This ground fault detector has particular applications in "floating" power supplies in which neither the positive nor the negative high voltage output terminal is referenced to chassis ground.
It is generally known in the art that ground fault detectors are utilized in high voltage apparatus to detect or measure leakage currents to ground. Once the leakage current reaches a pre-determined value, there is usually an alarm or warning system which is activated or else the high voltage apparatus is automatically disconnected to prevent a possible hazardous condition to personnel in the area of the apparatus. However, in many situations it would be undesirable to have the high voltage apparatus disconnected. For example, there are numerous laboratory experiments or tests requiring high voltage apparatus in which the voltage may rise during the test. In the ground fault detectors of the prior art, there is sensed the d.c. leakage current instead of an a.c. leakage impedance. Therefore, the alarm system would become activated in the middle of the experiment or test and thus not provide any advance warning at the beginning of the test. An automatic disruption would destroy the results of such experiments or tests. Under these circumstances, it would be more desirable to provide a ground fault detector which can supply a visual warning indication of a potentially dangerous or unsafe condition to personnel in the area at an operating voltage lower than that which could cause a hazardous condition.
Since these prior art d.c. ground fault detectors sense or measure only leakage current, they suffer another disadvantage in that they cannot provide a warning indication due to a dangerously large electrical capacitance which is capable of producing a lethal shock between a d.c. high voltage circuit and earth ground. Thus, it would be desirable to provide a ground fault detector which can detect a condition representing the presence of a potentially dangerous fault current or electrical charge by measuring continuously the a.c. electrical impedance between the d.c. high voltage circuit and earth ground. Once the a.c. electrical impedance decreases to a pre-selected low value, a visual warning indication will become activated.